Fluid-type flat-plate solar heat collector systems of a design employing panel units having a large number of parallel small-diameter tubes connected at their ends to larger-diameter header tubes are well known. In these systems water is pumped from a tank into the lower header tube and flows upwardly through the parallel tubes to the upper header tube and back to the tank through a return line. (In some designs the direction of flow is reversed). The tubes are bonded to a metal backing sheet, called the "plate". As both the plate and the tubes are of a conductive metal, usually copper, heat from the entire collector area is conducted to the water in the tubes, provided that the bond between the plate and tubes is highly conductive. Conductivity of the bond can vary as much as 1000 Btu per hour between a good bond and a poor one. Soldering is one of the common methods of bonding, but heat-conducting epoxy has also been used on recently developed flat-sided tube-type collectors. Because of the need for high conductivity, careful attention is required to the bonding procedure, introducing a considerable cost factor into the fabrication.
Along with utilization of collected solar heat for home heating or for heating other types of structures, electric power produced directly from sunlight is being used for household and other purposes. Cadmium sulfide (CdS) solar cells have been previously employed for this purpose, and more recently silicon-type solar cells have been developed which are highly efficient. However, the silicon cells, employed in the form of thin flat wafers, are relatively brittle and are subject to cracking when exposed to high temperature gradients, similar to the effect of a sudden temperature change caused by pouring hot liquid suddenly into a thin glass. If the water tubes are in close thermal contact with a hot solar heat-collecting plate, high temperature gradients will occur at the regions near the tubes when relatively cool water flows through the tubes.
In employing the solar cells, they are suitably cemented to the solar heat collection plate, using heat conducting but electrically insulating adhesive. By connecting groups of cells in series or parallel, almost any variety of power values can be obtained. Current from the cells is used to charge batteries that provide a direct-current supply day and night for lights. By employing inverters, alternating current may be produced for driving a variety of appliances.
At the present time there is a substantial need for providing a less expensive and more reliable fabrication technique for producing flat-plate solar heat collector units, and for providing an improved configuration which will definitely avoid high thermal stresses on electrical solar cells employed with the units.